The present invention relates to substrate coating applications and, more particularly, to multi-stage processes for coating a substrate with a first powder coating and a subsequent second powder coating, and the application of a combination of infrared radiation and convection treatment to at least one of the powder coatings.
Today""s automobile bodies are treated with multiple layers of coatings which not only enhance the appearance of the automobile, but also provide protection from corrosion, chipping, ultraviolet light, acid rain and other environmental conditions which can deteriorate the coating appearance and underlying car body.
The formulations of these coatings can vary widely. However, a major challenge that faces all automotive manufacturers is how to rapidly treat and cure these coatings with minimal capital investment and floor space, which is valued at a premium in manufacturing plants.
Various ideas have been proposed to speed up treating and curing processes for automobile coatings such as hot air convection treatment. While hot air treatment is rapid, a skin can form on the surface of the coating which impedes the escape of volatiles and entrapped air from the coating composition and causes pops, bubbles or blisters which ruin the appearance of the dried coating, and can mar or dislodge portions of the applied powder coating.
Other methods and apparatus for treating and curing a coating applied to an automobile body are disclosed in U.S. Pat. Nos. 4,771,728; 4,907,533; 4,908,231 and 4,943,447 in which the automobile body is heated with radiant heat for a time sufficient to set the coating on Class A surfaces of the body and subsequently cured with heated air.
U.S. Pat. No. 4,416,068 discloses a method and apparatus for accelerating the treating and curing of refinish coatings for automobiles using infrared radiation. Ventilation air used to protect the infrared radiators from solvent vapors is discharged as a laminar flow over the car body. FIG. 15 is a graph of temperature as a function of time showing the preferred high temperature/short treatment time curve 122 versus conventional infrared treatment (curve 113) and convection treatment (curve 114). Such rapid, high temperature treatment techniques can be undesirable because a skin can form on the surface of the coating that can cause surface defects, as discussed above. In the case of powder coating, the coating can xe2x80x9csetxe2x80x9d too quickly before adequate flow is achieved after melting. Melt viscosity and cure rate must be balanced to achieve optimum flow.
U.S. Pat. No. 4,336,279 discloses a process and apparatus for treating automobile coatings using direct radiant energy, a majority of which has a wavelength greater than 5 microns. Heated air is circulated under turbulent conditions against the back sides of the walls of the heating chamber to provide the radiant heat. Then, the heated air is circulated as a generally laminar flow along the inner sides of the walls to maintain the temperature of the walls and remove volatiles from the treatment chamber.
As discussed at column 7, lines 18-22, air movement is maintained at a minimum in the central portion of the inner chamber in which the automobile body is dried.
A rapid, multi-stage treatment process for automobile coatings is needed which inhibits formation of surface defects and discoloration in the coating, particularly for use with a first powder coating composition to be overcoated with a second powder coating composition.
The present invention provides a process for coating a metal substrate, comprising the steps of: (a) applying a powder first coating composition to a surface of the metal substrate; (b) applying a first infrared radiation at a power density of 30 kilowatts per meter squared or less and optionally a first air simultaneously to the first coating composition for a first period of at least about 90 seconds, a first temperature of the metal substrate being increased at a first rate ranging from about 0.3xc2x0 C. per second to about 1.25xc2x0 C. per second to achieve a first peak metal temperature ranging from about 125xc2x0 C. to about 200xc2x0 C., such that a sintered first coating is formed upon the surface of the metal substrate; (c) applying a second powder coating composition over the first coating; and (d) applying a second infrared radiation at a power density of 30 kilowatts per meter squared or less and a second air simultaneously to the second coating composition for a second period of at least about 2 minutes, a second temperature of the metal substrate being increased at a second rate ranging from about 0.8xc2x0 C. per second to about 1.3xc2x0 C. per second to achieve a second peak metal temperature of the substrate ranging from about 125xc2x0 C. to about 175xc2x0 C., such that a powder layered system is formed upon the surface of the metal substrate.
Another aspect of the present invention is a process for coating a substrate, comprising the steps of: (a) applying a first powder coating composition to a surface of the substrate; (b) applying a first infrared radiation at a power density of 30 kilowatts per meter squared or less and optionally a first air simultaneously to the first coating composition for a first period of at least about 90 seconds such that a sintered first coating is formed upon the surface of the substrate; (c) applying a second powder coating composition over the first coating; and (d) applying a second infrared radiation at a power density of 30 kilowatts per meter squared or less and a second air at an air velocity ranging from about 0.5 to about 13 meters per second simultaneously to the first coating composition for a second period of at least about 2 minutes, such that a powder layered system is formed upon the surface of the substrate.
Yet another aspect of the present invention is a process for coating a polymeric substrate, comprising the steps of: (a) applying a first powder coating composition to a surface of the polymeric substrate; (b) applying a first infrared radiation at a power density of 30 kilowatts per meter squared or less and optionally a first air simultaneously to the first coating composition for a first period of at least about 90 seconds, a first temperature of the polymeric substrate being increased at a first rate ranging from about 0.30xc2x0 C. per second to about 1.25xc2x0 C. per second to achieve a first peak polymeric temperature ranging from about 125xc2x0 C. to about 200xc2x0 C., such that a sintered first coating is formed upon the surface of the polymeric substrate; (c) applying a second powder coating composition over the first coating; and (d) applying a second infrared radiation and a second air simultaneously to the second coating composition for a second period of at least about 2 minutes, a second temperature of the polymeric substrate being increased at a second rate ranging from about 0.80xc2x0 C. per second to about 1.3xc2x0 C. per second to achieve a second peak polymeric temperature of the substrate ranging from about 90xc2x0 C. to about 175xc2x0 C., such that a powder layered system is formed upon the surface of the polymeric substrate.